\section{INTRODUCTION}

The knowledge of the dynamic characteristics of the propulsive system is very important to aircraf's control, in order to avoid undesired oscillations when throttle changes are suddenly applied to the aircraft. In fact, the throttle control is so important that it has been used successfully as the only control available to land a severely damaged aircraft as in \cite{AVIATION03}.

Also, all modern propulsive systems include a control system, such as a FADEC, to maximize the engine efficiency and to guarantee operation within certain limits in order to preserve the engine's service life.
In both situations cited above, a model of the dynamic response of the engine is essential.

In the present work, the main objective was to experimentally obtain the best dynamic model of a 0.61 cubic inches two-stroke piston engine. The engine was mounted on a test bed designed to measure the thrust and rotation per minute signals, acquired using LabView$^{\textregistered}$. The throttle lever was actuated on by a servo motor, thus controlling the carburetor's valve opening. The command signal was generated by LabView, processed by a micro-controller into a PWM format and sent to the servo motor.

In order to obtain the engine transfer function, two types of PWM input signals were used: a sinusoidal frequency sweep and a square wave (sequence of steps). For higher frequencies, the servo motor dynamics presented considerable influence on the resulting throttle lever angle input. Additionally, by reading the actual servo motor position from the voltage across an internal potenciometer, it was possible to obtain the dynamic model of the servo motor alone. Finally, the influence of the servo dynamics response was calculated in order to obtain the transfer function model of the engine alone.
